1. Field of the Invention
The present invention relates to a solid state photographing apparatus which is used to recognize an image as in the case of, for example, a digital camera and in which there are arrayed on a semiconductor substrate a plurality of cells each constructed using a combination of a photo-electric conversion element and transistors.
2. Prior Art
When roughly classified, small-sized photographing apparatuses are so done mainly into a type that each uses a CCD and a type that each uses a CMOS image sensor. Attention has nowadays been drawn toward a CMOS photographing apparatus (hereinafter referred to as “the CMOS image sensor”) that enables manufacturing a transistor part for driving a photo-electric conversion element, the use of that is needed for the photographing apparatus, by performing almost the same process steps as those for manufacturing an ordinary CMOS transistor. Further, compared with the CCD type, that CMOS photographing apparatus has the merit of its relevant portions' being able to be converted into a one-chip arrangement by being integrated on the same substrate when they are in the manufacturing process steps. The apparatus therefore has the merit of the drive's thereof being relatively small in power consumption. The apparatus further has the merit of being made small in size and low in cost.
However, compared with the CCD type, the CMOS type had the demerit of its being low in sensitivity. An example of a circuit structure of that CMOS image sensor is shown next. FIG. 30 is a plan view showing a conventional layout of the array wherein CMOS transistors are arrayed. As shown in FIG. 30, an active region is provided on a semiconductor substrate by means of element-isolating films thereon. The active region has at the center of the pixel a rectangular region as the photo-diode region and, further, has a substantially horizontally thrown U-shaped region connected thereto. The gate electrode of each CMOS transistor is formed in the way of crossing the active region of FD like the shape of the substantially horizontally thrown letter U. For enabling making great the saturated amount of electric charge of the photo-electric conversion element such as the photo-diode in that CMOS transistor arrangement, the occupied area of the photo-diode within the pixel is enlarged. By doing so, it is possible to enhance the sensitivity. However, in the above-mentioned circuit structure, the percentage of the area occupied by the photo-diode within the pixel is 29%, and, therefore, if the layout of the array of the CMOS transistors remains unchanged, it is difficult to make again higher the percentage of the area occupied by the photo-diode.
Also, by making large the size of the CMOS transistors within the pixel, especially the width of the gate electrodes thereof, it is possible to lessen the variation in characteristic of the CMOS transistors and, in consequence, to enhance the sensitivity of the CMOS image sensor. However, if the above-mentioned layout of the array of the CMOS transistors remains unchanged, it is difficult to make large the width of the gate electrodes.